The oxidation of polyunsaturated fatty substances constitutes a crucial problem as it modifies the quality of the oil and results in a bad taste of the oil, an unpleasant odour and a change of color.
Among the various reasons for the degradation of the oils, the presence of metals such as copper and iron have been recognized. These metals act indeed as powerful pro-oxidant.
It is therefore very important to eliminate the metals from the fatty substances. The metals are essentially present in the form of ions bound to phospholipids, which are themselves to be eliminated, such metals being typically iron, calcium and magnesium. These resulting salts are soluble complexes of phosphatidic acids and phosphatidylethanolamine. These complexes, also called nonhydratable phospholipids, are difficult to remove and traditional degumming is not sufficient for reducing the concentration below an acceptable level.
Numerous patents or patent applications are directed to processes for elimination of such non hydratable phospholipids.
U.S. Pat. Nos. 4,069,686 and 4,698,185 are typical examples. Phospholipids content may be reduced to around 20 ppm and iron content to 0.2 ppm. However these concentrations are still too high.
French patent 1,388,567 and PCT application WO 95/00609 disclose the refining of oil by using organic acids and emulsifiers. The fatty substance is in suspension in an aqueous phase containing the acid and the emulsifier. Phospholipids are eliminated as a result of complexation of polyvalent metals by acids, preferably polycarboxylic acids.
The presence of emulsifiers necessitates however further operations consisting for example in washings with centrifugation. On the other hand, the preferred emulsifiers, for example sodium laurylsulfate (SDS), is not allowed in some countries due to the difficulty of eliminating them at the refining step.
The purpose of the present invention is to overcome the mentioned disadvantages and to provide an economically valuable process for eliminating completely metals such as calcium, magnesium, iron from fatty substances, animal or vegetable fat, crude or delecithinated. After separation of the aqueous phase, the iron content is indeed brought down to 8 to 0.05 ppm. Also the phosphorous content originally ranging from 800 to 100 ppm may be reduced to less than 5 ppm even for poor quality oils.
According to the present invention, there is provided a process for removing phospholipids and/or polyvalent metals from a fatty substance comprising the step of mixing the fatty substance with an aqueous solution of a salt of polycarboxylic acid characterised in that the mixture is operated in such a way to form only , or essentially only, fine droplets or micelles of said aqueous phase in the fatty substances. No detergent or emulsifier is added previously or simultaneously to the mixing step.
The elimination of metals and bound phospholipids is achieved by intimate mixing of an aqueous solution containing a polycarboxylic complexing agent in the forms of droplets, emulsion or micelles in an excess of the oil to be refined.
The complexing agent is preferably an acid comprising at least 3 carboxylic functionalities, in the acid form or in the form of salts of monovalent cations. The preferred complexing agent is a sodium salt of ethylenediaminetetraacetic acid (EDTA). A potassium salt or a mixed salt may also be appropriate.
The sodium EDTA salts solution used in the process may also be a mixture of sodium EDTA salt with any other inorganic base.
According to one embodiment of the invention, the fatty substance containing a high proportion of non hydratable phospholipids is first intimately mixed with an organic or inorganic acid in such a way to dissociate the phospholipid-metal complexes and to facilitate the complexation of the metals with a aqueous solution of sodium EDTA intimately mixed to the oil. Simultaneously, the sodium EDTA solution neutralizes the free hydroxyl functionalities of phosphatidic acid (P.A.) and of phosphatidylethanolamine (P.E.) in order to make them hydratable as sodium salts.
The preferred organic acid used to dissociate the phospholipid-metal complex is selected from the group consisting of citric acid, malic acid, ethylenediaminetetraacetic acid, tartric acid, oxalic acid, maleic acid or an inorganic acid such as phosphoric, hydrochloric or sulfuric acid.
The concentrated acid is added in low quantities to the fatty substances. If it is added as a concentrate, the amount of acid may vary from 0.005 to 0.15% by weight relative to the oil. However it may be diluted up to 90%. This pre-treatment with an acid is advantageously achieved at the same temperature than the one of the addition of the complexing agent water solution. In some cases, a different temperature is chosen.
According to the above embodiment, the acid is mixed for a very short time, typically 3-4 seconds to 10 minutes, with a high-shear mixer or a homogenizer. It may also be mixed with a traditional stirrer for more than 10 minutes.
Preferably, the aqueous solution of complexing agent comprises also an electrolyte derived from monovalent cations for example NaCl, KCl or Na2SO4. The concentration of the electrolyte will vary preferably between 0.1% and 10%, typically between 0.5 and 5 % by weight.
The amount of complexing agent is near the equivalent stoichiometric amount of the estimated amount of metals in the oil to be refined.
The EDTA salt chelates efficiently the metallic polyvalent cations (F++, Fe+++, Ca++, Mg++) and forms with the latter far more stable complexes than the ones obtained with phosphatidic acid or even phosphoric or citric acids which have a lower stability constant. Consequently, the nonhydratable phospholipid-metal complex (M=FE++, Ca++, Mg++) is rapidly displaced by the sodium salt of EDTA in order to provide on one part a new hydratable EDTA-M complex and on the other part an hydratable sodium phospholipid salt. The EDTA complex is very stable, even at high temperature. The separation of the aqueous phase may then be achieved at an higher temperature by decantation or centrifugation, with consequential low losses in neutral oils and a high separation capacity.
The oils containing a high proportion of metals and phospholipids treated in that way may be bleached in a classical way with equivalent amounts of earth than the ones necessary in the chemical refining process and may be physically refined in order to generate oils of better quality as regards resistance to oxydation, and this at competitive prices.
The elimination of metals and phospholipids in accordance with the invention may be easily achieved at the location of extraction, for example in the palm oil mill, where the sludge oils are eliminated and so produce an oil which require only one stage of deacidification-deodorisation by steam stripping. The invention provides therefore a substantial economical advantage.
The intimate mixture of xe2x80x9caqueous phase in oilxe2x80x9d is obtained for example by using a high-shear mixer or homogenizer of the Ultraturax type between 50 and 20,000 rpm or by using a ultrasonic device. Typically the aqueous solution is between, 0,5 to 30% by weight, preferably 3 to 15%, more preferably 5 to 10% by weight of the fatty substance to refine. The latter has most often already been degummed in a classical way for removing the hydratable phospholipids by delecithination, that is to say by stirring in hot water and centrifugation.
The reaction is preferably performed at a temperature varying from 5 to 120xc2x0 C., more preferably around 80xc2x0 C.
The reaction time will depend on the temperature. The reaction may take a few seconds, a few minutes or more than one hour.
As already mentioned, the preferred chelating (complexing) agent is ethylenediaminetetraacetic acid or one of the corresponding sodium, potassium or ammonium salt. This acid will preferably be in the di-sodium, tri-sodium or tetra-sodium form.
As already mentioned, the amount of complexing agent in the aqueous phase is chosen by taking into account the estimated amount of metals to be removed and the presence of free carboxylic acids in the oil to be refined, as well as the nature and the form of the complexing agent. This amount is advantageously around the stoichiometric amount, varying from 1 to 2, preferably 1.2 to 1.5 equivalent. For example for the tetrasodium salt of EDTA and a vegetable oil already degummed using a traditional method, there must be taken into account the fact that the free carboxylic acids will react first with the tetrasodium EDTA salt to provide the corresponding salt and tri-sodium EDTA. The latter will then complex the metals bound to the phospholipids.
According to one aspect of the invention, the fish oils and the rapeseed oils which contain large amounts of metals phospholipids and sulphur compounds may be desulphurised simultaneously to the process of the invention.
According still another aspect of the invention, the crude maize and sunflower oils with high iron, phospholipids and wax contents may be dewaxed simultaneously with the present process. After intimate mixing of the aqueous phase with the crude oil, the suspension is cooled under weaker stirring between 5 and 90xc2x0 C., preferably, 5 and 50xc2x0 C., before centrifugation. There is obtained in that way a purified wax-free oil with a iron content lower than 0.05 ppm, and the phosphorous content is lower than 5 ppm. Advantageously, an emulsifier may be added to the initial mixture during the cooling process.
Analogously, a palm oil may be treated by an aqueous solution containing the same reactants, then cooled progressively in a cristalliser before centrifugation. A light phase appears, comprising palm olein from which metals, phospholipids and glycolipids have been removed, and a heavy phase, containing stearin cristals surrounded by the aqueous phase.
The latter phase is heated and centrifuged in order to provide palm stearin also without metals, phospholipids and glycolipids. Using such process, palm oil may be degummed, demetallised and fractioned in one and the same operation.
The process of the invention allows the physical refining of oils with phosphorous content lower than 50 ppm or even lower than 20 ppm, without addition of bleaching earth or with an amount of earth lower than 0,5%, in order to provide an oil of maximum red 3 (Lovibond 5xc2xcxe2x80x3) in the case of palm oil.
More particularly, the palm oil will more efficiently cristallise, will be more easily fractioned and the yield will be higher.
Waxes are easily partially or completely removed from crude or delecithinated oils.